Page:Elementary Text-book of Physics (Anthony, 1897).djvu/271

§ 336] was developed; while in the third case the motor, causing the tread-wheel to revolve, ;performed work, which produced heat in addition to that due to the chemical action.

It has been thought that muscular energy is due to the waste of the muscles themselves; but experiments show that the waste of nitrogenized material is far too small in amount to account for the energy developed by the animal; and we must, therefore, conclude that the principal source of muscular energy is the oxidation of the non-nitrogenized material of the blood by the oxygen absorbed in respiration.

An animal is, then, a machine for converting the potential energy of food into mechanical work: but he is not, as Mayer supposed, a heat-engine; for he performs far more work than could be performed by a perfect heat-engine, working between the same limits of temperature, and using the food as fuel.

The food of animals is of vegetable origin, and owes its energy to the solar rays. Animal heat and energy are, therefore, the transformed energy of the sun.

The tides are mainly caused by the attraction of the moon upon the waters of the earth. If the earth did not revolve upon its axis, or, rather, if it always presented one face to the moon, the elevated waters would remain stationary upon its surface, and furnish no source of energy. But as the earth revolves the crest of the tidal wave moves apparently in the opposite direction, meets the shores of the continents, and forces the water up the bays and rivers, where energy is wasted in friction upon the shores or may be made use of for turning mill-wheels. It is evident that all the energy derived from the tides comes from the rotation of the earth upon its axis; and a part of the energy of the earth's rotation is, therefore, being dissipated in the heat of friction it causes.

The internal heat of the earth and a few other forms of energy, such as that of native sulphur, iron, etc., are of little consequence as sources of useful energy. They may be considered as the remnants of the original energy of the earth.

236. Energy of the Sun.—It has been seen that the sun's rays